The present invention relates to bonding wires for use in fabricating semiconductor devices and to a method for producing such bonding wires. Particularly, the invention relates to ball-type bonding wires used in forming connections to electrodes of semiconductor chips.
Gold has been widely employed as the material of bonding wires used in semiconductor devices. However, since gold is expensive and since the long-term reliability of connections between a gold wire and an aluminum electrode on a semiconductor chip is relatively low, it has been proposed to use instead copper, aluminum, palladium, or certain alloys of these elements. Particularly, an aluminum alloy containing 2.0% magnesium and an aluminum alloy containing 1.0% silicon are recognized as acceptable, as disclosed in Johnson et al., "Ultrasonic Wire Welding", Solid State Technology, vol. 20, pages 91-95, April 1977, and Gehman et al., "Aluminum Wire for Thermosonic Ball Bonding in Semiconductor Devices", Solid State Technology, vol. 26, pages 151-158, October 1983.
As disclosed in these articles, in order to eliminate the directional restriction of the wire when connected to the electrode of the semiconductor device, a tip of the metal wire is shaped into a ball. The shaping of the tip of the wire is usually performed by applying a high d.c. voltage between the tip of the wire and a consumable electrode to form an electric discharge therebetween and thereby melt the tip and shape it to a ball by surface tension. In such a case, a positive potential is applied to the electrode and a negative potential is applied to the wire in view of ease of insulation breakdown.
FIG. 1a illustrates ball formation at the tip of a metal wire 1 made of copper or aluminum using the same method as that used for ball formation at the tip of a gold wire. In FIG. 1a, the metal wire 1, supported by a capillary chip 5 which is also used as a bonding tool, is disposed opposite a consumable electrode 2 in an inert gas atmosphere 7 of a gas such as argon. A d.c. source 4 is connected between the wire 1 and the electrode 2 with the positive and negative terminals of the d.c. source 4 being connected to the electrode 2 and the wire 1, respectively. An arc 3 is thereby formed between the wire 1 and the electrode 2. Due to the discharge, the tip portion of the metal wire 1 is heated and melted.
Thermal electron emission at the negative potential side tends to occur in areas whose work function is smaller than other areas, that is, in areas in which more stable emission than other areas is possible. Since the wire 1 of copper or aluminum has a naturally formed oxide film several tens of angstrom thick on its surface, the area where thermal electron emission occurs tends to expand, as shown by the hatched in FIG. 1a. Therefore, heat is applied to such a widened area, resulting in a defective ball 8 having a non-melted portion 1a formed around the tip of the wire 1, as shown in FIG. 1b. Such a non-melted portion 1a causes minute cracks to be produced in a silicon chip when the defective ball 8 is bonded to one of its pads, even if such a portion exists invisibly within the ball 8. (There is no such problem for gold wire because no oxide film forms on a gold wire.)
Further, when copper wire is used, to provide acceptable bonding characteristics to an electrode of a semiconductor device, the wire should have a hardness compatible with that of the electrode of the semiconductor device, which pad is usually made of aluminum. The Vickers hardness of an aluminum pad is about 30 to 40, which is acceptable for gold wire. However, since ordinary copper has a Vickers hardness of 60 or more, it generally has not been preferable to use copper for bonding wires.
Further, it has been known that the shear strength of an aluminum pad formed by vapor deposition of highly pure aluminum is as low as 5 to 7 kg/mm.sup.2, and, when a wire of copper of the like is bonded to such pad and the semiconductor device molded in resin, breakage or deformation of the aluminum pad may occur during the molding process. That is, if the molding resin is of the thermoplastic type, the molding temperature is about 300.degree. C., and if it is a low temperature resin, the molding must be performed under a high pressure. In such cases, the conventional aluminum pad, having a low shear strength, cannot withstand the high temperature or high pressure.